1. Field of the Invention
The present invention relates to a liquid crystal spacial light modulator used in display panels, image information recording etc., and to a holographic image information recording apparatus using the same.
2. Description of the Prior Art
Researches on image information recording and reproduction techniques using holography have been under way since the 1960s. Especially, three-dimensional information recording and reproduction apparatus have recently begun to attract attention from the fields of medicine, art, etc. as the application value of such apparatus is recognized. Active matrix liquid crystal display (LCD) panels, for use in portable TV, projection-TV, etc., using twisted nematic liquid crystals have also been actively developed, and high performance, high resolution products have begun to be marketed. Under these circumstances, the holographic image recording system has been proposed which uses an LCD panel as the spacial light modulator (SLM) and displays a computer-processed image on the liquid crystal SLM to produce object light.
In the holographic image recording system, the performance of the liquid crystal SLM plays an important part in determining the quality of recorded images. Besides, anti-noise measures are an essential requirement, which is not the case with conventional LCD panels illuminated with incoherent light. To specifically describe the measures, it is required to suppress speckle noise, interference noise, etc., which are caused as a result of the illumination of the liquid crystal SLM by coherent light.
In a conventional LCD panel, as shown in FIG. 1, polarizing plates 2a and 2b are respectively attached to the front and back surfaces of a liquid crystal device 1. Light emitted from a light source 10 is passed through a lens 20, a pinhole 23, and a lens 21, and is polarized by the polarizing plates 2a and 2b in the rectilinear direction coinciding with the anchoring direction of the liquid crystal device 1. The liquid crystal device 1 rotates the direction of polarization of the incident light to modulate the intensity of the outgoing light emitted through the polarizing plate 2b on the back side. However, such a construction is disadvantageous in that the performance, including the contrast ratio, of the liquid crystal device 1 cannot be fully utilized, being restricted by the performance of the polarizing plates 2a and 2b.
To describe in detail, the polarizing plates have the shortcomings in that the light transmittance is low, that the extinction ratio is relatively low, and that the polarizing direction is not uniform but dispersed within the surface. Furthermore, there is a possibility that the polarizing direction may slightly deviate from the proper angle because of an error in attaching the polarizing plates to the liquid crystal device. As a result, the contrast ratio of the SLM is reduced. Another factor contributing to the reduced contrast ratio is the inability to obtain the proper polarizing direction for incident light other than that of a particular wavelength because the proper polarizing direction with the liquid crystal device 1 varies depending on the wavelength of the light entering the SLM. As a result, the contrast ratio of the SLM is reduced.
Further, in the conventional LCD panel, film-like polarizers are used. The problem is that the light is reflected on the surface of the polarizers, thus generating interference noise. To prevent the reflection of light, an anti-reflection film can be provided on the surface of the polarizer, but it is difficult to provide a durable anti-reflection film on the surface of a film-like polarizer.
For the above reasons, in order to obtain a high quality image using a liquid crystal SLM, it is necessary to adjust the polarizing direction of the incident light without providing a polarizer on the front side of the liquid crystal panel. To achieve this purpose, it is possible to collimate the linearly polarized light from a laser light source for direct entry to the liquid crystal device, but in such a construction, if the laser light source itself is to be rotated through a minute angle for fine adjustment of the polarizing direction of the laser beam, a relatively large-sized mechanism will be required for the rotation of the laser light source. On the other hand, a mirror may be used so as to reduce the size of the entire optical system, but the light reflected by a mirror generally becomes elliptically polarized and is therefore not desirable. Furthermore, when the liquid crystal SLM is applied to holography, the problem is that the reference light separated from the object light by a half mirror also becomes elliptically polarized. Thus, in the liquid crystal SLM, it is not easy to properly design an optical system capable of fine-adjusting the polarizing direction of the incident light.